Plasma polymerization, as well as numerous other techniques, have been used to prepare membranes for filtration purposes. The membranes formed by the process consist of substrates upon which are deposited coatings of polymeric compounds. The composite membranes thus formed act as selective filtering devices. Known substrate materials include porous glass and ceramics, and a variety of organic polymer materials which will readily support the polymeric compound coating which is deposited thereon to form the desired membrane.
U.S. Pat. No. 3,775,308 discloses a semipermeable composite membrane formed by electrodeless glow discharge polymerization of an organic monomer in the presence of a porous support. This patent teaches the combination of various porous support materials with organic monomers to yield a membrane which possesses filtration characteristics. The membranes therein are utilized as reverse osmosis membranes in desalination devices.
U.S. Pat. No. 4,032,440 accomplishes the same type of membrane preparation as just mentioned. This patent, however, combines both an organic monomer and an inorganic monomer with a support material to yield a composite membrane for reverse osmosis consisting of a porous support substrate coated with a copolymer.
These membranes, however, as well as other conventionally prepared membranes, demonstrate a tendency to foul during the filtration process, which has been an industry recognized concern. As used herein, the term "foul" refers to the blocking and plugging of membrane pores during the filtration process. Fouling is due to membrane polarization and the formation on the membrane surface of sub-layers of particles and other compounds present in the filtration system. The build-up of these sublayers causes reduced flux rates and eventually clogs the membrane pores completely.
A further contributor to flux rate loss, common in the filtration of biological systems, is foaming and frothing. Antifoam agents are often used at various stages of processing to control foam problems. Antifoam agents are, however, generally hydrophobic and therefore tend to concentrate on interfaces, thus contributing to the fouling of the membrane. The adsorption of the antifoam onto the membrane surface causes increased reduction in flux rates and may even cause increased macromolecule retention.
There is a present need for a means of reducing the fouling of ultrafiltration and microfiltration membranes. Further, in systems where antifoam agents are utilized, the need is even more pressing, as such agents tend to quickly and irreversibly foul membranes, thus causing increased flux reduction and correlative loss in efficiency.
It is one object of the present invention, therefore, to provide a process by which membranes used for ultrafiltration/microfiltration purposes can be protectively coated such that fouling is substantially reduced.
It is a further object of the present invention to provide a protective membrane coating which allows for continued optimum flux rate of the membrane even after the addition of antifoam agents to the filtration system.
These and other objects of the present invention will become apparent to one skilled in the art from a reading of the following description of the invention and the appended claims.